It has a batterycompartment on the back. I think it measures a little bigger than my cameras spotmetering, and thatīs good. I want to measure the light in the room and not just one spot. Do you think I can put a white bulb-like thing on top off the "hole" ontop of the meter so it measures less "spot"-like, if you know what I mean? More like a modern lightmeter...

The "bulbs" are for incident readings: You directly measure the light falling on the subject by holding the meter in front of it and pointing the meter towards the camera.

There's nothing especially modern or not modern about incident meters, just most "modern" reflective meters will also allow you to use them in incident mode.

If your meter doesn't have a "bulb" or "diffusor" slide built in, you can certainly attach one of your own, but you may want to compare the result with a reflected reading of an 18% (neutral) grey card, in case your "bulb" absorbs a little light.

But do read up on the differences and relative advantages of reflected and incident metering...
(Cue usual statements of how incident/spot/zone system/matrix/averaging/center-weighted metering is the *only* way to go).